Discrete wavelength tunable laser

ABSTRACT

A discrete wavelength tunable laser having an optical cavity which comprises: a reflective semiconductor optical amplifier (SOA); a demultiplexer (Demux) having a single input and a plurality of outputs, the Demux configured to receive the output of the SOA and to produce a plurality of fixed spectral passbands within the gain bandwidth of the SOA; one or more tunable distributed Bragg reflector(s) (DBR(s)) arranged to receive the outputs of the Demux, each tunable DBR configured to select a reflective spectral band within the gain bandwidth of the SOA upon application of a bias current; wherein the SOA forms the back end mirror of the optical cavity; the one or more tunable DBRs form the front end mirror of the optical cavity; and wherein the lasing channel of the discrete wavelength tunable laser is chosen by the overlap of the selected reflective spectral band of one of the one or more tunable DBRs with a fixed spectral passband of the Demux.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a National Phase Patent Application and claimspriority to and the benefit of International Application NumberPCT/GB2017/050408, filed on Feb. 17, 2017, which claims priority toBritish Patent Application Number 1602947.2, filed on Feb. 19, 2016; theentire contents of both of the documents identified in this paragraphare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a discrete wavelength tunable laser,particularly to a discrete wavelength tunable laser having an opticalcavity which includes a combination of a passive optical grating and oneor more digital supermode-distributed Bragg reflector(s).

BACKGROUND OF THE INVENTION

Continuously tunable lasers are well-established and are commonplace intelecommunications applications. Although telecommunications lasersoperate to fixed grids (eg ITU grids), tunable lasers need to be set upfor a variety of applications and some extent of wavelength tunabilityis desirable to allow for correction of wavelength drift as the laserages. Unfortunately, the requirement for full range continuoustunability results in expensive and power-hungry electronic circuitry,most particularly due to the requirement for digital-analog conversion(DAC) chips.

Distributed Feedback (DFB) lasers in which the gratings are built intothe gain medium are being replaced by Distributed Bragg Reflector (DBR)lasers, particularly where tunability is required. For a wide range oftunability a Sampled Grating (SG) DBR laser is one typical option. Insuch a laser, the grating (often referred to as a “comb grating”) givesrise to a comb of reflectivity peaks which can be tuned to select therequired lasing wavelength.

In an alternative design of tunable laser, Digital Supermode DBRs(DS-DBRs) may be utilised. The DS-DBR design has the advantage overSG-DBR in that no DACs are required. However, tunable lasers madeentirely on semiconductor chips have been impossible without gratingsrequiring DACs for control. For example, no tunable laser has been madewith solely DS-DBR gratings (i.e. no other types of gratings). Thechallenge of this patent application is to create tunable lasers basedupon the DS-DBR design principle but with cheaper and lower powerconsuming control electronics, in particular not requiring DACs.Semiconductor lasers, made absent DACs for primary control, aredisclosed herein. This is achieved principally by devising finite statetunable devices.

The application of the AWG as a wavelength selective device for laserarrays is known (Keyvaninia et al, Optics Express, 21 (No 11) p 13675,30 May 2013).

SUMMARY OF THE INVENTION

Accordingly, the present invention aims to solve the above problems byproviding, according to a first aspect, a discrete wavelength tunablelaser having an optical cavity which comprises: a semiconductor opticalamplifier (SOA); a wavelength demultiplexer (Demux) having a singleinput and a plurality of outputs, the AWG configured to receive theoutput of the SOA and to produce a plurality of fixed spectral passbandswithin the gain bandwidth of the SOA; one or more tunable distributedBragg reflector(s) (DBR(s)) arranged to receive the outputs of theDemux, each tunable-DBR configured to select a reflective spectral bandwithin the gain bandwidth of the SOA upon application of a bias current;wherein the SOA forms the back end mirror of the optical cavity; the oneor more tunable-DBRs form the front end mirror of the optical cavity;and wherein the lasing channel of the discrete wavelength tunable laseris chosen by the overlap of the selected reflective spectral band of oneof the one or more tunable-DBRs with a fixed spectral passband of theDemux.

The one or more tunable-DBRs are configured to select a given spectralpassband from the Demux which corresponds to the chosen mode of thelaser (i.e. to the desired output of the wavelength) by reflecting thereflective spectral band which corresponds to the desired wavelength.The optical cavity of the tunable laser is therefore formed between aback end mirror on the SOA and a front end mirror formed by the one ofthe tunable DBRs to which a bias current is provided.

A single SOA can therefore provide the light for all channels of thetunable laser. This means that the wavelength tunable laser of thepresent invention achieves wavelength switching without the need formultiple SOAs. Unlike previous prior art examples where there exists aneed to switch between multiple SOAs, there is no requirement in thetunable lasers described herein for complicated driving circuits toswitch SOAs on and off. Instead, the entire tunable laser can beoperated by a simple driving circuit which drives the one or moretunable DBRs directly.

Optional features of the invention will now be set out. These areapplicable singly or in any combination with any aspect of theinvention.

The Demux may take the form of any component suitable for demultiplexingthe output of the SOA according to the wavelength. This Demux may takethe form of an Arrayed Waveguide Grating (AWG). Other forms of Demuxinclude: a Planar Concave Grating (PCG), a ring resonator array, aninterleaver structure, a multimode interference device MMI, or acascaded Mach Zehnder interferometer.

Each of the tunable DBRs may be a digital supermode distributed BraggReflector (DS-DBR).

Each of the one or more tunable DBRs may be a phase-tunable DistributedBragg Reflector which preferably includes a phase tuning region, thecarrier density of which can be manipulated by application of a currentor voltage bias. The bias may be a forward bias or a reverse biasdepending on the properties of the phase tuning region chosen. Adjustingthe bias will in turn adjust the phase and position in frequency spaceof the reflectance spectrum or transmittance spectrum of that region ofthe DBR. The phase tuning region may be a portion of or all of the DBR.

Optionally, the phase tuning region includes a p-n junction device. Inthis way, the p-n junction device provides an electrically tunable phasetuning region whereby the phase of the reflectance spectrum of the DBRcan be adjusted via carrier depletion by application of a varyingreverse bias. The p−n junction may optionally be a p+−p−n−n+, orp++−p+−p−n−n+−n++ structure.

The AWG may be fabricated integrally with other waveguides on a singleSOI chip. Alternatively, the AWG may be fabricated as a separate chipand aligned with other waveguides on the silicon chip during assembly.

The discrete wavelength tunable laser may include a single waveguidewhich optically couples the output of the SOA to the input of the AWG;and may include further waveguides, each of which is optically coupledto a respective output of the AWG; each of the further waveguidesincluding a respective one of the one or more tunable DBRs.

In this way, the AWG receives light from the SOA via a single waveguide.The AWG then transmits light corresponding to each respective fixedspectral passband via a respective waveguide, each respective waveguidecomprising a tunable DBR.

Optionally, the AWG is a 1×N AWG which splits the spectral output of theSOA into an integer number N of respective output waveguides; and thediscrete wavelength tunable laser may include a further AWG which actsas a multiplexer to multiplex the signals from the N respective outputwaveguides to produce a common output of the laser.

Optionally, the AWG is a first 1×M AWG which splits the spectral outputof the SOA into an integer number M of respective output waveguides;wherein the discrete wavelength tunable laser includes a further AWGwhich acts as a multiplexer to multiplex the signals from the Mrespective output waveguides to produce a common output of the laser;and wherein each respective waveguide includes one or more tunable DBRsthe one or more tunable DBRs on each waveguide having a total number ofP reflecting wavelength states.

Optionally, the AWG is a cyclic AWG wavelength router, having at least Pcycles.

Optionally, the 1×M AWG has a channel spacing of Δf and an FSR of MΔf.

Where the AWG is a 1×M cyclic router, the first output waveguide wouldreceive spectral passbands from the AWG corresponding to wavelengthvalues λ₁, λ_(M+1), etc. up to λ_((P−1)M+1). The tunable DBR gratings onthat waveguide will therefore be configured (by way of the gratingseparation) to reflect wavelength states with wavelength values λ₁,λ_(M+1), etc. up to λ_((P−1)M+1). The second output waveguide wouldreceive spectral passbands corresponding to wavelength values λ₂,λ_(M+2), etc. up to λ_((P−1)M+2). The tunable DBR gratings on thatwaveguide will therefore be configured (by way of the gratingseparation) to reflect wavelength states with wavelength values λ₂,λ_(M+2), etc. up to λ_((P−1)M+2). The final output waveguide (the M^(th)output) would receive spectral passbands from the AWG corresponding towavelength values λ_(M), λ_(2M), etc. up to λ_((P−1)M+M) (i.e. λ_(PM),).It can therefore be understood that the total number of discretewavelength states that the tunable laser can be tuned to is given byM×P.

Optionally, the AWG is a non-cyclic 1×P AWG which splits the spectraloutput of the SOA into an integer number P of respective outputwaveguides; wherein the discrete wavelength tunable laser includes afurther AWG which acts as a multiplexer to multiplex the signals fromthe P respective output waveguides to produce a common output of thelaser; and wherein each respective waveguide includes one or moreDS-DBRs the one or more tunable DBRs on each waveguide having a totalnumber of M reflecting wavelength states.

Where this 1×P AWG is non-cyclic AWG, the first of the P outputwaveguides would receive spectral passbands from the AWG correspondingto wavelength values λ₁, λ₂, etc. up to λ_(M). The tunable DBR gratingson that waveguide will therefore be configured (by way of the gratingdesign) to reflect wavelength states with wavelength values λ₁, λ₂, etc.up to λ_(M). The second output waveguide would receive spectralpassbands corresponding to wavelength values λ_(M+1), λ_(M+2), etc. upto λ_(2M). The tunable DBR gratings on that waveguide will therefore beconfigured (by way of the grating design) to reflect wavelength stateswith wavelength values λ_(M+1), λ_(M+2), etc. up to λ_(2M). The finaloutput waveguide (the P^(th) output) would receive spectral passbandsfrom the AWG corresponding to wavelength values λ_((P−1)M+1),λ_((P−1)M+2), etc. up to λ_((P−1)M+M) (i.e. λ_(PM),). As with the cyclicexample, it can therefore be understood that the total number ofdiscrete wavelength states that the tunable laser can be tuned to isgiven by M×P.

Optionally, the Arrayed Waveguide Grating (AWG) (i.e. the AWG which isconfigured to receive the output of the SOA and to produce a pluralityof fixed spectral passbands within the gain bandwidth of the SOA)includes: a coarse tuning AWG in the form of a 1×P AWG, the coarsetuning AWG having one input which is optically coupled to the SOA and Poutputs, the 1×P AWG configured to output a sub-region of the SOAspectral passband to each of its P respective outputs; a plurality offine tuning AWGs, where each fine tuning AWG is a 1×M AWG; the input ofeach of the 1×M AWG being optically coupled to one of the P respectiveoutputs of the 1×P AWG; M output waveguides respectively coupled to theM outputs of each fine tuning AWG to give a total of M×P outputwaveguides from the 1×M AWGs; and a further AWG which acts as amultiplexer to multiplex the signals from the M×P respective outputwaveguides to produce a common output of the laser; wherein each of theM output waveguides of each of the fine tuning AWGs comprises a tunableDBR grating.

Optionally, the discrete wavelength tunable laser of claim 1, furthercomprises additional P SOAs and additional P AWGs such that the tunablelaser includes: a plurality of SOAs and a plurality 1×M AWGs, the outputof each SOA providing an input to a 1×M AWG; a plurality M of outputwaveguides optically coupled to the respective M outputs of each 1×MAWG, wherein each of the M outputs of each 1×M AWG includes a tunableDBR grating. Optionally, each 1×M AWG may have a channel spacing of Δfand an FSR of MΔf.

It is envisaged that the AWG could be replaced by any other passiveoptical component configured to produce a comb-like transmittancespectrum. For example, suitable alternatives could include a ringresonator, a waveguide Fabry-Perot filter or a Multimode Interference(MMI) device configured to act as a Fabry-Perot filter.

It is also envisaged that the tunable DBRs (which may take the form ofDS-DBRs) could be adapted to compensate for the spectral profile of thegain medium. The gain element will have a large spectral range overwhich the power of light generated will depend upon the wavelength.Usually there will be less power at the two extremes of the spectralrange, creating a “drop off” in power at the edges of the range. TheDBRs could be adapted to compensate for such drops in gain. For example,the reflectivity of the DBR could be reduced at parts of the DBR whichcorrespond to high-gain regions of the spectral profile of the gainmedium. Alternatively, or in addition, the reflectivity of DBRs could beincreased at sections configured to reflect wavelengths which correspondto spectral regions of low-gain from the gain medium.

Optionally, for any one of the aspects above, the discrete wavelengthtunable laser may further comprise one or more phase tuner(s) for finetuning the wavelength of the laser.

This phase tuner would be separate from any phase tuners that may formpart of the tunable DBR(s). The fine tuning phase tuner may be used toaccount for drift due to temperature or other environmental factors.

In any of the embodiments described herein, said mirror located at theback end of the semiconductor gain medium may preferably have areflectivity of at least 85% and even more preferably, the mirror has areflectivity of at least 90%. A standard high reflectivity coating maybe applied to give the desired reflectivity over the desired bandwidth.

In some embodiments, the SOA (Semiconductor Optical Amplifier) is anRSOA (Reflective Semiconductor Optical Amplifier). In this way, the RSOAforms the back mirror of the optical cavity.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1a shows a schematic diagram of a discrete wavelength tunablelaser;

FIG. 1b shows a schematic diagram of spectral profiles of the AWG andDBR gratings of the embodiment shown in FIG. 1 a;

FIG. 2a shows a schematic diagram of an alternative discrete wavelengthtunable laser;

FIG. 2b shows a schematic diagram of an alternative discrete wavelengthtunable laser;

FIG. 2c shows a schematic diagram of spectral profiles of the AWGs andDS-DBRs of the embodiment shown in FIG. 2 b;

FIG. 3 shows a schematic diagram of an alternative discrete wavelengthtunable laser; and

FIG. 4 shows a schematic diagram of an alternative discrete wavelengthtunable laser.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES OF THE INVENTION

A first embodiment of a discrete wavelength tunable laser 100 isdescribed with reference to FIGS. 1a and 1b . The discrete wavelengthtunable laser 100 includes a semiconductor optical amplifier (SOA)reflective semiconductor optical amplifier (RSOA) 101 which generateslight over a given gain bandwidth. The back surface of the RSOA 102includes a high reflectivity mirror at 102 forming back end mirror ofthe optical cavity of the tunable laser.

The tunable laser includes an Arrayed Waveguide Grating (AWG) 105 in theform of a 1×N AWG which has a single input optically coupled to theoutput of the RSOA via a waveguide 104. A phase tuner 103 is located atthe waveguide 104 for fine tuning of the wavelength.

The AWG has N outputs, each of which transmits or passes a respectivefixed spectral passband, each of which lies within the gain bandwidth ofthe RSOA.

A plurality N of waveguides are each optically coupled to a respectiveone of the N outputs of the 1×N AWG. Each output waveguide thereforecorresponds to a particular wavelength channel of the AWG.

Each of the N output waveguides includes a tunable distributed Braggreflector (DBR) grating 106 and each DBR is configured to be able toselect the spectral passband of that waveguide (cause a back-reflectionwithin the spectral passband of that waveguide) upon application of abias current.

The reflection peak of each tunable DBR is slightly detuned from thewavelength channel for the waveguide 106 at which it is located when nobias is applied. When a bias is applied to the DRB grating to selectthat channel, the resulting phase change means that the reflection peakof the grating becomes tuned at or near to the center wavelength of thatchannel, and the grating becomes the front end mirror of an opticalcomprising that waveguide, creating a laser. The lasing channel (i.e.one of the possible lasing modes of the laser) of the discretewavelength tunable laser is therefore chosen by the overlap of thereflection band of a given DBR with a fixed spectral pass-band of theAWG.

Each of tunable DBRs includes a pn junction and electrical connectionsincluding a drive circuit and drive pins which provide an electricalcontact between the drive circuit and the pn junction. By applying abias from the drive circuit, the DBR is configured such that it can beswitched to its “on” wavelength by application of a single set voltage.In the “off” state the wavelength of the gratings 106 corresponds with ahigh-loss off-band wavelength of the AWG 105. In the “on” state thereflection wavelength of the DBR corresponds to one of the spectralpass-bands of the AWG. The resulting overlap enables the optical cavityto lase at the selected wavelength. Only one of the gratings 106 will beon at a given time. When a DBR 106 is “selected”, it forms the front endmirror of the optical cavity.

The activation of a required wavelength channel (i.e. the lasingwavelength of the laser) can be seen in more detail in FIG. 1b in whichthe required channel “I” is activated because the transmission spectrumof the ith one of the N passbands of the AWG overlaps spectrally withthe reflection spectrum of the selected one of the N DBR gratings.

The output of the laser light is directed to a single output waveguideby connecting the channelized output waveguides to a multiplexer. InFIG. 1 this multiplexor takes the form of an N×1 AWG 107 although othertypes of multiplexers may be used.

The phase tuner 103 provides fine tuning and therefore seeks to providea mechanism to counter wavelength drift over the lifetime of the tunablelaser. In the example shown in FIG. 1, N may be 48, the gratings 106 mayhave reflectivity of 10%, and the channel spacing of the AWG may be 100GHz. The advantage of this embodiment is that one commercially availableRSOA powers all channels, for example all 48 channels. In general thetolerance to aging of this design is also advantageously high.

The AWG of this embodiment and the AWGs of embodiments described belowmay be fabricated integrally with the other features (waveguides) or maybe fabricated as a separate chip and aligned during assembly with thewaveguides on the silicon chip.

The AWG 105 may be an integrated part of the optical chip (100) or maybe a separate device. The optical chip 100 may optionally be fabricatedin silicon such as silicon-on-insulator.

Two further embodiments of discrete wavelength tunable lasers 300 a and300 b are described below with reference to FIGS. 2a and 2 b.

Each discrete wavelength tunable laser 300 a, 300 b comprises an RSOA301 with a highly reflective back facet 302.

In the embodiment 300 a shown in FIG. 2a , the RSOA is optically coupledto a 1×M AWG 305 and the M output waveguides of this AWG include DS-DBRgratings located therewith. In FIG. 2a , the AWG 305 is a cyclic AWGwith at least P cycles, has a channel spacing of Δf, where Δf is thetunable laser grid channel spacing, and an FSR of M*Δf. The DS-DBRgratings on each of the waveguides 1 to M are fabricated to have Preflecting wavelength states, the first waveguide having wavelengthvalues 1, M+1, 2M+1, etc up to (P−1)M+1, the second waveguide havingvalues 2, M+2, 2M+2, etc. up to (P−1)M+2, and the last waveguide havingvalues M to PM (since (P−1)M+M=MP) where P is N/M, and N is the numberof total wavelength states of the tunable laser.

For example, if P=7, there would be 7 grating wavelengths available perwaveguide, and 7 sections to each DS-DBR grating. In other words, therewould be 7 grating wavelengths available to be selected on for eachspectral passband of the AWG. If M is 7, then there are 49 total modescorresponding to 49 wavelength channels available for the tunable laser.When a wavelength is selected on a DS-DBR, the DS-DBR becomes reflectiveat that wavelength so that the optical cavity of the laser is formedbetween the reflector 302 of the RSOA and the selected DS-DBR.

The embodiment shown in FIG. 2b differs from that of 2 a in that the 1×MAWG 305 of FIG. 2a is replaced by a non-cyclic 1×P AWG 305 b.

A non-cyclic AWG can be advantageous in that the losses for the “worstcase channel” of the AWG can be made to be lower. The transmissionlosses through the channels at the edges of an AWG's FSR are typicallyhigher, and in a cyclic AWG the channels at the edges of the FSR must beused. With a non-cyclic AWG, the FSR can be designed to be significantlylarger than the bandwidth of the channels that are used, so that thechannels that are used are in the center of the FSR and therefore have alower loss.

On the other hand, the use of cyclic AWGs can be advantageous overnon-cyclic AWGs because when using a non-cyclic AWG for this purpose theindividual channel transmission bands must each have pass band width ofM*Δf, and the P pass bands must pass all P*M channels, therefore thepassbands must have transmission spectra that are close to square-shaped(thereby leading to a constant loss across all wavelengths being passed,and high isolation of all other wavelengths). For example, AWG channel 1must pass all sub-channels 1 to M with little loss variation, and rejectall other sub-channels, and AWG channel 2 must pass all sub-channels M+1to 2M with little loss variation, and reject all other sub-channels.However such difficulties can be mitigated if the wavelength grid usedby the system is allowed to have gaps between each group of M wavelengthcombs.

In the embodiment of FIG. 2b , the AWG 305 b is a P-channel AWG with achannel spacing of MΔf, and a 3-dB channel transmission spectrumbandwidth sufficient to pass M channels of Δf channel spacing. TheDS-DBR gratings on waveguides 1 to P are fabricated to have M reflectingwavelength states, the first waveguide having wavelength values 1, 2, .. . M, the second having values M+1, M+2, . . . M+P, the third waveguidehaving wavelength values 2M+1, 2M+2, . . . 2M+P, and the last waveguidehaving wavelength values (P−1)M+1, (P−1)M+2, . . . (P−1)M+M, which isequal to PM.

FIG. 2c shows example transmission spectra of the P AWG channels of theAWG outputs shown in FIG. 2b , each output channel of the AWG having aspectral range “W” which lies within the bandwidth of the SOA. FIG. 2calso shows example reflection spectra of the M-section DS-DBR gratingson each of the P waveguides.

The selection of the wavelength channel M−2 is shown. This occurs whensection M−2 of a DS-DBR is selected on the first waveguide by a biasvoltage applied to electrode M−2 on the DS-DBR grating on the waveguideof AWG channel 1. In this way, the laser mode M−2 is selected out of M×Pdiscrete modes available for this discrete-mode tunable laser. In someembodiments (not shown), rather than selecting the desired lasingchannel by applying a bias directly to the section of the DBRcorresponding to that channel, the section may be “selected” by applyinga bias of the opposite polarity to a section immediately adjacent to thedesired section.

An alternative discrete wavelength tunable laser 400 is described belowwith reference to FIG. 3. The embodiment shown in FIG. 3 differs fromthat shown in FIGS. 1a and 1b in that the AWG is actually made up of twoseparate AWG stages; a first coarse AWG stage 405 and a second finetuning AWG stage 409. The overall transmission function of thecombination of AWGs 405 and 409 is the same or similar to thetransmission function of AWG 105 in FIG. 1.

The coarse tuning AWG 405 takes the form of a 1×P AWG, having one inputwhich is optically coupled to the RSOA on its input side and opticallyconnected to a plurality P of output waveguides on its output side. The1×P AWG itself passes a range of spectral passbands across each of its Prespective outputs.

The fine tuning AWGs 409 each take the form of a 1×M AWG. The input ofeach of the 1×M AWG is optically coupled to one of the P respectiveoutputs of the 1×P AWG and the M outputs are each optically coupled toan output waveguide. Each of the M output waveguides includes a tunableDBR grating.

Unlike the embodiments of FIGS. 2a and 2b , each selectable wavelengthchannel of the AWG has its own single-section grating. Thesingle-section DBRs are much simpler to manufacture than the DS-DBRgratings of FIGS. 2a and 2b . The size of the 1×M and 1×P AWGs combinedcan be made to be smaller than a single 1×N grating. Therefore the sizeof the device of this embodiment can be less than that in FIG. 1, butwill still be larger than those in FIGS. 2a and 2 b.

In an alternative embodiment of FIG. 3, the location of the coarse andfine AWGs may be swapped so that AWG 405 is a cyclic AWG and gives afine wavelength selection, in the same way as the 1×M cyclic AWG in FIG.2a , and AWGs 409 give coarse wavelength selection.

The discrete wavelength tunable laser 400 includes a further N×1 AWG 407which acts as a multiplexer to multiplex the signals from the M×Prespective output waveguides to produce a common output of the laser.

FIG. 4 shows a schematic diagram of an embodiment containing multipleRSOAs coupled to multiple respective AWGs.

For the same discrete wavelength states for the tunable laser asprevious embodiments, the lasing cavities are divided amongst aplurality P of RSOAs, each RSOA having the same gain bandwidth in thecase where the RSOAs are an array of RSOA waveguides on a single chipcoming from the same wafer, or, alternatively, having different gainbandwidths optimized for its corresponding downstream AWG, in the casewhere the ROSA are different chips. In the embodiment shown, an RSOA 201generates optical power for M lasing wavelengths using a downstream 1×MAWG. Each 1×M AWG is designed to pass a comb with a channel spacing ofM*Δf, the first AWG passing wavelength values 1, 2, . . . M, the secondpassing values M+1, M+2, . . . 2M, the third passing wavelength values2M+1, 2M+2, . . . 3M, etc., and the last waveguide having wavelengthvalues (P−1)M+1, (P−1)M+2, . . . PM. The 1×M AWGs are thus designed thesame as the AWGs 409 in FIG. 3.

As in the previous embodiments, the M outputs from the lasers from allof the AWGs 205 are combined using an N×1 multiplexer 207, which acts tomultiplex the signals from the M×P respective output waveguides toproduce a common output of the laser.

Compared with the embodiment in FIG. 3, the first 1×P AWG is replacedwith RSOAs, thus the optical path length of the laser cavities isshorter and so tuning speed is faster and laser mode spacing larger, butthis is achieved at the cost of a plurality of RSOAs. Additionally, ifthe laser is to be power efficient, only the RSOA that provides gain forthe laser mode being selected should be powered on during the time thatmode is selected, and all other RSOAs should be powered off. This addsadditional complexity to the tuning electronics. For a 49-wavelengthoutput laser, P could be 7 and the number of RSOAs would be 7. In thisembodiment the whole device could be integrated on one chip or it couldbe constructed from several chips. The chip and chips could be SOI.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

For example, in some embodiments described herein, the AWG is used togive a wavelength filter function (i.e. as a wavelength demultiplexer).However, AWGs with coarse filter functions can be difficult to designand manufacture. As an alternative, Mach-Zehnder (MZ) interferometerssuch as Mach-Zehnder waveguide interferometers may be used. Whenconstructed in a cascade, with differing phase changes in the arms ofeach MZ in the cascade, such MZs can be made into a useful coarse filterwith square-like pass-band shapers. Echelle gratings could also be usedin place of any of the AWGs.

Whilst the embodiments described herein all comprise a reflectivesemiconductor optical amplifier (RSOA), it is envisaged that they couldall be carried out using a standard SOA (without a mirrored surface). Inthis case, the SOA would be a double sided structure and both sideswould need to be coupled to the SOI host chip. A separate rear mirror(not part of the SOA) would need to be fabricated to be opticallycoupled to the back side of the SOA.

All references referred to above are hereby incorporated by reference.

The invention claimed is:
 1. A discrete wavelength tunable laser havingan optical cavity which comprises: a semiconductor optical amplifier(SOA); a wavelength demultiplexer (Demux) having a single input and aplurality of outputs, the Demux configured to receive the output of theSOA and to produce a plurality of different fixed spectral passbandswithin the gain bandwidth of the SOA; one or more tunable distributedBragg reflector(s) (DBR(s)) arranged to receive the outputs of theDemux, each tunable DBR configured to select a reflective spectral bandwithin the gain bandwidth of the SOA upon application of a bias current;and a back end mirror, wherein the one or more tunable DBRs form thefront end mirror of the optical cavity; wherein the SOA is between theback end mirror and the Demux; and wherein the lasing channel of thediscrete wavelength tunable laser is chosen by the overlap of theselected reflective spectral band of one of the one or more tunable DBRswith a fixed spectral passband of the Demux.
 2. The discrete wavelengthtunable laser of claim 1, wherein: a single waveguide optically couplesthe output of the SOA to the input of the Demux; the discrete wavelengthtunable laser comprises further waveguides, each of which is opticallycoupled to a respective output of the Demux; and wherein each of thefurther waveguides includes a respective one of the one or more tunableDBRs.
 3. The discrete wavelength tunable laser of claim 1, wherein theDemux is an Arrayed Waveguide Grating (AWG).
 4. The discrete wavelengthtunable laser of claim 3, wherein: the AWG is a 1×N AWG which splits thespectral output of the SOA into an integer number N of respective outputwaveguides; and wherein the discrete wavelength tunable laser includes afurther AWG which acts as a multiplexer to multiplex the signals fromthe N respective output waveguides to produce a common output of thelaser.
 5. The discrete wavelength tunable laser of claim 3, wherein: theAWG is a first 1×M AWG which splits the spectral output of the SOA intoan integer number M of respective output waveguides; the discretewavelength tunable laser includes a further AWG which acts as amultiplexer to multiplex the signals from the M respective outputwaveguides to produce a common output of the laser; and wherein eachrespective waveguide includes one or more tunable DBRs the one or moretunable DBRs on each waveguide having a total number of P reflectingwavelength states.
 6. The discrete wavelength tunable laser of claim 5,wherein the first 1×M AWG is a cyclic AWG wavelength router.
 7. Thediscrete wavelength tunable laser of claim 5 wherein the first 1×M AWGhas a channel spacing of Δf and an FSR of MΔf.
 8. The discretewavelength tunable laser of claim 3, wherein: the AWG is a non-cyclic1×P AWG which splits the spectral output of the SOA into an integernumber P of respective output waveguides; wherein the discretewavelength tunable laser includes a further AWG which acts as amultiplexer to multiplex the signals from the P respective outputwaveguides to produce a common output of the laser; and wherein eachrespective waveguide includes one or more tunable DBRs, the one or moretunable DBRs on each waveguide having a total number of M reflectingwavelength states.
 9. The discrete wavelength tunable laser of claim 3,wherein the Arrayed Waveguide Grating (AWG) configured to receive theoutput of the SOA and to produce a plurality of fixed spectral passbandswithin the gain bandwidth of the SOA is a plurality of AWGs including: acoarse tuning AWG in the form of a 1×P AWG, having one input which isoptically coupled to the SOA and P outputs, the 1×P AWG configured tooutput a range of spectral passbands to each of its P respectiveoutputs; a plurality of fine tuning AWGs, where each fine tuning AWG isa 1×M AWGs; the input of each of the 1×M AWG being optically coupled toone of the P respective outputs of the 1×P AWG; M output waveguidesrespectively coupled to the M outputs of each fine tuning AWG to give atotal of M×P output waveguides from the 1×M AWGs; and a further AWGwhich acts as a multiplexer to multiplex the signals from the M×Prespective output waveguides to produce a common output of the laser,wherein each of the M output waveguides of each of the fine tuning AWGscomprises a respective one of the one or more tunable DBRs.
 10. Thediscrete wavelength tunable laser of claim 1, wherein the SOA is areflective semiconductor optical amplifier (RSOA) comprising the backend mirror.
 11. The discrete wavelength tunable laser of claim 3,further comprising additional SOAs and additional AWGs such that thetunable laser includes: a plurality of SOAs and a plurality 1×M AWGs,the output of each SOA providing an input to a 1×M AWG; and a pluralityM of output waveguides optically coupled to each 1×M AWG, each of the Moutput waveguides optically coupled to a respective one of the M outputsof that 1×M AWG, wherein each of the M outputs of each 1×M AWG includesa respective one of the one or more tunable DBRs.
 12. The discretewavelength tunable laser of claim 1, wherein each of the one or moretunable DBR(s) is a digital supermode-distributed Bragg reflector(DS-DBR).
 13. The discrete wavelength tunable laser of claim 11, whereinthe plurality of SOAs are located on a single chip.
 14. The discretewavelength tunable laser of claim 11, wherein each of the plurality ofSOAs are located on a different chip.
 15. The discrete wavelengthtunable laser of claim 11, wherein the plurality of SOAs are reflectivesemiconductor optical amplifiers (RSOAs).